Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 29 2022 16:01:32 FOUS30 KWBC 291601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Thu Dec 29 2022 Day 1 Valid 16Z Thu Dec 29 2022 - 12Z Fri Dec 30 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL RANGES OF SOUTHERN OREGON, NORTHERN CALIFORNIA AND PORTIONS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY... 16Z Update... Few changes needed to the outlook in either parts of the country.=20 Radar showed showers developing over western/northern Louisiana close to the axis of the previously-issued Slight Risk Made only minor adjustments to the northern and western portion of the Slight Risk based on those radar trends and the corridor of greatest QPF from the models. Along the West Coast...one batch of rainfall was making its way southeastward across California while the next push of moisture approaches the coast and should enter a region of difluence in the upper flow between the subtropical jet and a northern stream system. Bann ....West... Flash flooding concerns return to the West Coast as a pair of low pressure systems arrive today. The first system will bring a round of light rainfall to the coasts of the Pacific Northwest and northern/central California early this morning. Rain rates should be around 0.1in/hr across the region with PWATs remaining modest at well below an inch. Forcing provided by a weak surface cold front won't help to enhance amounts, but higher totals will still be possible at higher elevations. The second round of precipitation will be a bit more substantial and will arrive with the entrance of another system later Thursday afternoon. Rain rates will be a little higher at around 0.1-0.25in/hr. PWATs between 0.75-1 inch should cause some flash flooding concerns over already well saturated soils, burn scars and urban areas. ....Lower Mississippi Valley... An upper trough over the Great Plains will cause height falls and the return of southerly flow over the Gulf of Mexico and Mississippi Valley. Moist air advection will ensue with PWATs of between 1-1.5 inches flowing from the central Gulf coast up through the Mississippi Valley and into the Midwest. This moist/unstable airmass, overrunning a coastal warm front, a dry line and cold front to the west, will support convective initiation over the Gulf coast and inland areas, where rain rates may range between 0.25-1in/hr with locally higher amounts. The latest 00z guidance is in good agreement with respect to the orientation and magnitude of the axis of greatest QPF in the Southern Plains/Lower Mississippi Valley. Kebede Day 2 Valid 12Z Fri Dec 30 2022 - 12Z Sat Dec 31 2022 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER THE COASTAL RANGES OF NORTHERN CALIFORNIA, THE WESTERN FOOTHILLS OF THE SIERRA NEVADA, AS WELL AS PARTS OF THE CENTRAL GULF COAST AND SOUTHEAST... ....West... Heavy rainfall continues across portions of the West Coast on Friday. A shortwave impulse embedded within an upper ridge will gather enough Pacific moisture to cause flash flooding concerns over coastal portions of northern/central California, including the Bay Area. Two rounds of rainfall associated with two separate low pressure systems will occur on Friday and the early hours of Saturday morning, respectively. Rain rates of 0.1-0.50in/hr are anticipated with both rounds. PWATs between 0.75-1.25in are also expected with anomalies between +3 and +4. The highest rain rates and anomalies are expected along the northern coast and western foothills of the Sierra, where upslope enhancement should contribute to higher totals. Guidance has trended wetter since last night's issuance, therefore the northern California coast may see increase to between 2-4 inches and the Sierra foothills may accumulate between 3-5 inches by early Saturday morning. Western portions of Nevada, including the Reno metro area, have been included in the marginal risk area due to high snow levels and antecedent rainfall causing vulnerable surfaces to be more susceptible to runoff/flooding. The slight risk area mostly highlights places that are already high in soil moisture, burn scars and urban areas. ....Central Gulf Coast & Southeast... An upper trough stationed over the Great Plains will continue focusing moisture from the Gulf of Mexico at the Central Gulf Coast and inland areas on Friday. There's reasonable agreement regarding the axis of heaviest rainfall (1-3 inches) setting up along the Gulf Coast from eastern Louisiana to the Florida Panhandle and western Georgia. PWATs between 1.5-2 inches and hourly rain rates ranging from 0.25-1in/hr may cause isolated instances for flash flooding. The heaviest rates will likely be within convective cells. Another stripe of QPF maxima is expected to develop along a northern stream system and associated cold front over portions of the Lower/Central Mississippi Valley into the Midwest but amounts around an inch in 24 hours only warrant a marginal risk at this time. Fortunately, higher flash flood guidance and low stream levels will mitigate a lot of the heavy rainfall impacts. Kebede Day 3 Valid 12Z Sat Dec 31 2022 - 12Z Sun Jan 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COASTAL CALIFORNIA AND THE FOOTHILLS OF THE SIERRA NEVADA... The latest guidance seems to be in reasonable agreement with respect to Friday's AR event over the northern/central coast of California sagging south through central California on Saturday and into southern Cali by the time the new year rolls around on Sunday morning. An upper trough will amplify as it enters the West on Saturday. The 00z GFS is a bit deeper and slightly more progressive than the 00z EC with the amplification of this upper trough, and that is reflected in the higher QPF totals across northern/central California and northern Nevada. The 00z EC is a bit wetter than the 00z GFS in southern California, though. 0.1-0.25in/hr rain rates are expected across the California coast and Sierra foothills while PWATs are likely to be around 1 inch or 2-3 standard deviations above average. Two day totals in northern California ending at 12z Sunday morning are forecast to be in excess of 4 inches, while the Sierra foothills are expected to receive over 7 inches. Inland valleys may receive a couple of inches as well. Parts of southern California are likely to receive between 1-2 inches of rainfall, which could be enough to cause flash flooding over burn scars and urban areas. The day 2 marginal risk area was continued over portions of western Nevada on day 3 due to the continued rainfall threat, particularly through Saturday morning. The marginal risk area over portions of the central Gulf coast and Southeast is to account for high two day totals in parts of the Southern Appalachians and eastern Alabama from Friday's heavy rainfall event and additional rainfall for portions of northern Florida up into the Southern/Central Appalachians. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6pKQa6yTt6ZyNo5ceSXD4ot_V5flkLFZ64HR5lpIsQYq= WGscc7Yi7-xy003QwAOmASZplwXyOzKx6MtGOVRpaS9bV7w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6pKQa6yTt6ZyNo5ceSXD4ot_V5flkLFZ64HR5lpIsQYq= WGscc7Yi7-xy003QwAOmASZplwXyOzKx6MtGOVRprv8FDcU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6pKQa6yTt6ZyNo5ceSXD4ot_V5flkLFZ64HR5lpIsQYq= WGscc7Yi7-xy003QwAOmASZplwXyOzKx6MtGOVRpWNYIU1I$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .